/**
  * Begin nesting predicates
  *
  * @return Predicate
  */
 public function nest()
 {
     $predicateSet = new Predicate();
     $predicateSet->setUnnest($this);
     $this->addPredicate($predicateSet, $this->nextPredicateCombineOperator ?: $this->defaultCombination);
     $this->nextPredicateCombineOperator = null;
     return $predicateSet;
 }